Abstract: The goal of R01 HL85440 is to explore the cellular events and molecular signals involved in repair and remodeling of the infarcted myocardium. In the previous cycle we have identified key molecular signals that inhibit pro-inflammatory macrophage activation, while promoting myofibroblast transdifferentiation and repair of the infarcted heart. The current proposal focuses on the role of cardiac pericytes in regulation of post-infarction inflammation, fibrosis and angiogenesis in the infarcted heart. The proposed studies will test the hypothesis that in the healing infarct, activation of distinct growth factor-mediated pathways modulates phenotype of cardiac pericytes, promoting pericyte to fibroblast transdifferentiation during the proliferative phase of infarct healing, while regulating inflammation and angiogenesis through interactions with endothelial cells. This novel hypothesis will be tested in 3 specific aims: Aim 1: To characterize cardiac pericytes in normal and infarcted mouse hearts and to explore the fate of pericytes in the infarcted myocardium using lineage tracing approaches. Our preliminary data using NG2DsRed pericyte reporter mice show that: a) normal mouse hearts contain a large population of pericytes, b) following myocardial infarction the cardiac pericyte population undergoes a series of dynamic changes. During the early inflammatory phase of infarct healing, there is a marked reduction in pericyte density in the infarcted zone, followed by accumulation of NG2+ cells with myofibroblast characteristics in the infarct border zone. Later, during the maturation phase of infarct repair, pericytes are recruited by neovessels and may participate in regulation of the angiogenic response. Accordingly, we will study the phenotypic characteristics of pericytes during the phases of cardiac repair, and we will perform lineage tracing experiments to study the contribution of pericytes to the fibrotic and angiogenic response. Aim 2: To investigate the role of platelet-derived growth factor receptor (PDGFR) signaling in guiding pericyte phenotype and function in the infarcted heart. PDGF signaling critically regulates phenotype and function of vascular mural cells in vitro and in vascular development. Our experimental work suggests that: a) the receptors PDGFR? and PDGFR? are upregulated in the infarcted myocardium, and b) in an experimental model of reperfused myocardial infarction, PDGFR? antibody neutralization attenuates collagen deposition, whereas PDGFR? inhibition perturbs healing, interfering with vascular maturation and coating of infarct neovessels with mural cells. We will use pericyte-specific PDGFR knockdown strategies in vivo, and in vitro experiments to explore the distinct effects of PDGFR? and PDGFR? signaling in regulating pericyte phenotype and function in the infarcted myocardium Aim 3: To examine the role of TGF-? signaling on pericyte fate in the healing infarct. Although a large body of in vitro evidence suggests that pericytes are highly responsive to TGF- ? stimulation, the in vivo effects of TGF-? signaling in pericytes remain unknown. In order to investigate the role of TGF-? signaling in pericytes in the infarcted myocardium, we will perform in vivo experiments in animals with conditional pericyte-specific disruption of TGF-?/Smad signaling and in vitro studies using cultured mouse cardiac pericytes and endothelial cells.